Rotating hearth electric furnace



April 15, 1941. MacPHAIL 2,238,799

ROTATING HEARTH ELECTRIC FURNACE Filed Aug. 28, 1959 7? 3 MVM ATTORNEYS Patented Apr. 15, 1941 2,238,799 ROTATING nma'rn ELECTRIC summon Arthur A. MacPhail, Midland, Mich., assig'nor to The Dow Chemical Company, Midland, Mich a corporation of Michigan Application August 28, 1939, Serial No. 292,189

7 Claims.

This invention relates to a rotating hearth electric furnace for use in metallurgical processes, particularly those operated under vacuum.

In various metallurgical processes, such as the production of calcium and magnesium, an oxidecontaining ore and a reducing agent are heated together at an elevated temperature, usually above 1000" C., at which temperature they interact, liberating a metal vapor, which may be withdrawn and condensed, and leaving a solid residue. A number of furnaces have been suggested for use in such thermal reduction processes, but have been far from ideally suited to the purpose. In particular, the furnaces have not provided for an even heating of the reduction charge under controlled conditions of temperature and reaction time.

In view of these difficulties, an object of the invention is to provide an improved electric furnace for use in thermal reduction processes of the character mentioned. Another object is to describe a furnace in which reduction charges may be heated evenly at reaction temperatures for carefully controllable periods of time. Other objects will be apparent from the following description.

The construction of the improved furnace may best be explained with reference to the accompanying drawing in which Fig. 1 is a vertical sectional view of a furnace according to the invention, and

Fig. 2 is a horizontal sectional view along the line 2-2 in Fig. 1.

In the preferred embodiment illustrated, the furnace consists of a cylindrical gas-tight steel shell ll provided with a removable cover l2 and supported above the floor on legs l3. The furnace chamber or reaction zone I4 is formed of a vertical cylinder 15 of graphite or other refractory material, and a circular cover it of the same material, and is mounted on a cylindrical base I! of refractory brick resting on the bottom of the shell H. Within the furnace chamber I4 is a rotating hearth structure consisting of a thick circular horizontal plate or hearth l8 of diameter only slightly smaller than that of the cylinder l and preferably made of the same material as the latter, this plate being supported by a refractory brick base l9 mounted on a circular steel plate 20. The entire hearth structure is supintroduced into the furnace chamber from a hopper 24 through a vacuum lock 25, a screw conveyor 2, and a feed pipe 21, and drops onto the hearth It. At the temperature of the furnace, the charge reacts, forming metal vapors which escape through an outlet 28 into a condenser 29 projecting through the shell H and provided with a removable cover 30. Residue remaining after reaction of the ore charge is removed from the hearth l8 by a rotatable toothed wheel or scraper 3| which is mounted just above the hearth 18 at the periphery thereof at a. position which is angularly remote, in the direction of hearth rotation, from the feed pipe 21. The scrapermay be rotated by a shaft 32 which extends upwardly out of the furnace I 4 through a gas-tight bearing 33 in the cover l2 to driving means not illustrated. When rotated, the scraper 3| forces reaction residue of! the edge of the hearth into a graphite chute 34 from which it spills into a pit 35 having a gas-tight removable cover 38.

The furnace chamber 14 is heated by electric resistor bars 31 which are disposed radially a short distance above the hearth I8. One end of each resistor bar is secured to a central graphite terminal 38 which is insulated from the cover 16 by an alundum bushing 39 and is connected to a graphite bus-bar 40 extending outside the shell II through a gas-tight joint 4| to a terminal 42. The other end of each resistor bar 31 is connected firmly to the graphite wall 15 of the chamber [4, this wall being electrically grounded on the shell H at a terminal 43 by means of a graphite rod 44.

Gas may be admitted to the furnace chamber through valved inlets l5, l6, and 41 which lead, respectively, into the feed pipe 21, the residue pit 35, and the bottom of the chamber ll below the hearth structure, as hereinafter explained. The furnace is evacuated through a valved gas outlet 48 located in the cover 30 of the condenser 29. In practice, the space between the furnace chamher and the steel shell is filled with heat insulating material.

In operating the furnace, the hearth l8 and its supporting structure is rotated at a slow' rate about the shaft 22, and the screw conveyor 26 and the scraper 3! are set in motion. Electric current of the desired voltage is applied to the terminals l2 and 43, thereby heating the resistor bars 31 and raising the furnace to an operating temperature. A charge of oxide ore and reducing agent is placed into the hopper 24, from which it is fed slowly into the feed pipe 21 and falls onto the rotating hearth I8 as a uniform thin layero The rotation of the hearth carries the charge directly under the resistor bars Sll, which heat it quickly to a reaction temperature. Metal vapors are evolved rapidly and escape to the condenser where they are chilled by contact with. the condenser Walls, forming solid metal. Reaction residue left on the hearth is carried around to the rotating scraper and is there removed into the pit After the furnace has been in operation for some time, the comdensecl metal. ancl the residual slag are retrieved by temporarily shutting the furnace clown, discontinuing the feed, releasing the vacuum, and removing the covers from the condenser aria slag pit or so as to permit scraping out the condenser and pit, @peratiou is thee. resumed.

The furnace chamber ill is continu ously through the gas outlet in the condenser 29 by means of a suitable vacuum pump not shown. In, order to instn'e that the metal vapors liberated in the furnace escape only into the coudenser, small quantities of an inert gas are bled continuously into the reaction chamber through the inlets 65, so, and W. In this way there is at all times a small positive vapor flow from all parts of the furnace toward. the condenser, metal vapors do not clifiuse into the iced; ll or pit 35 or around the hearings 2i where they might otherwise condense, causing opening di culties and loss of metal.

It will be appreciated that in the furnace lllu trated, the period of expo-sure of the reduction charge to the elevated temperature oi the furnace may be regulated, simply by adjusting the rota tional speed of the hearth. Careful time-tem= perature control oi the reduction process is thus easily attained. In addition, the charge is rtezl by radiation While in the form oi a thin layer; heat transfer conditions are ideal and uniform and complete reaction throughout the entire charge is achieveri, Continuous automatic trouble-free operation is entirely possible.

It is to be understood that the invention is not limited to the specific embodiment shown. Thus, the hearth be in the form of an annular ring instead of being a soliolplate, in which case the reaction residue may be scraped off at the center of the hearth rather than at the edge. The valved inlets t5, Gil, and Gil may be replaced with other vapor-directing means, and, for some operations, entirely eliminated. Qther changes in detail within the scope of the appended claims will doubtless occur to those skilled in the art.

if claim:

i. Ina high-temperature vacuum electric lurnace for use in processes wherein an oxide ore charge is heated, liberating a metal vapor and leaving a solid residue, the combination of a gastight shell; a furnace chamber formed of hightemperature refractory material mounted within the shell; a rotatable hearth structure within the chamber; means for evacuating the furnace; elec tric resistance heating elements disposed within the furnace chamber directly above the hearth and radially about a point above the center thereof a condenser in communication with the cham" her and having a gas outlet for applying vacuum to the chamber; a residue pit in communication with the chamber; mechanical feeding means for introducing an ore charge into the furnace chanc bet onto the rotating hearth; a vacuum lock in said feeding" means; and scraping means adapted to scrape residue from the hearth into the residue it 2, in a high-temperature vacuum electric iur= mace for use in processes wherein an. oxide ore charge is heated, liberating a metal vapor and leaving a solid residue, the combination of: a gas-= tight shell; a graphite furnace chamber mounted into the furnace chamber onto the rotating hearth; a vacuum lock. in said feeding means; anol a rotatable toothed wheel mounted directly above the hearth and adapted to scrape residue into the aforesaid chute.

3. In a high-temperature vacuum electric furmace, the combination: a gas-tight shell; a furnace chamber formed of a graphite cylinder surmounted by a circular graphite plate, the said cylinder being mounted on refractory brick restinc on the bottom of the shell; a hearth structure within the furnace chamber comprising a circular graphite hearth having a diameter only slightly smaller than that of the graphite cylintier; means for rotating; the hearth structure; and means for evacuating the furnace.

in a lugh-teu oerature vacuum electric furnace, the combin ion or: a gw-tight shell; a furmace chamber formed oi a graphite cylinder surmolmted by a circular graphite plate, the said cylinder bolus mounted onrefractory brlcl: resting on the bottom the shell; a rotatable hearth structure within. the furnace chamber comprising a circular graphplate having a diameter only slightly smaller than. that of the graphite cylinder and mounted on refractory hriclr resting upon a circular steel plate, said plate resting on hearings mounted. on the bottom oi the gas-[tight shell; and means for evacuating the furnace.

5. In a high-temperature vacuum electric furnace for use in processes wherein an oxide ore charge is heated, liberating metal vapor and leavthe e, residue, the combination of: a gas tight shell; a'iuruace chamber formed of a high termperature refractory material mounted ithito the shell; a rotatable hearth structure filll the chamber; means for evacuating the furnace; elec tric resistance heating" elements disposed within the furuace chamber directly above the hearth amt radially about a point above the cent there of a condenser in communication with the cham ber; means for introducing an ore else" e the furnace chamber onto the hearth; and means for removing residue from the hearth.

6. In a high-temperature vacuum electric fur mace, the combination of a gas tight shell; iur

nace chamber mounted Within the shell and formed of a cylinder of high-temperature refractory material surmounted by a circular refractory plate; electric heating means Within the lur mace chamber; a hearth structure within the tur nece chamber-comprising a circular refractory hearth having a diameter only slightly smaller than that oi the refractory cylinder; means for rotating the hearth structure; and means for evacuating the furnace.

l. in a high temperature vacuum. electric iur mace for use in processes wherein an oxide ore charge is heated, liberating a metal vapor and a solid residue, the combination of z a gas tight shell; a graphite furnace chamber mounted posed near the rotating hearth; feeding means for introducing an ore charge into the furnace chamber onto the rotating hearth; a vacuum lock in said feeding means; and a scraper mounted directly above the hearth and adapted to scrape residue into the aforesaid chuite.

ARTHUR A. MACPHAIL. 

